Coating that selectively absorbs radiation, and method thereof for achieving ambient temperature

ABSTRACT

The instant invention relates to thermal applications. More specifically, it relates to coatings applied on metals, used for harnessing solar radiation or artificial illumination. The object of this coating is to improve the efficacy of solar energy collection, maximizing visible light harnessing and minimizing heat emission in the metal. The procedure including the composition and obtainment of the solar absorbing coating operating in the low and medium temperature range from 25° C. to 300° C., and that can be used in devices generating heat through solar radiation or artificial illumination is described.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage entry of PCT/MX2014/000173 filedNov. 4, 2014, under the International Convention claiming priority overMexican Patent Application No. MX/a/2014/001213 filed Jan. 29, 2014.

FIELD OF THE INVENTION

The instant invention relates to thermal applications. Morespecifically, it relates to a selective absorbing coating used on metal,employed for harnessing solar energy and converting it into thermalenergy. The object of said coating is to improve the efficiency in thecollection of thermal energy, optimizing visible light harnessing andminimizing heat emission in the metal.

OBJECT OF THE INVENTION

The object of the instant invention is to present a process forobtaining a solar radiation selective absorbing coating at roomtemperature operating in the low and medium temperature range from 25°C. to 300° C. that can be applied on devices generating heat throughsolar radiation or artificial illumination. It also relates to differentuses and applications that can be given to it.

BACKGROUND OF THE INVENTION

The object of the selective absorbing coatings is to increase theefficiency of solar collectors and are generally used in thermosolarapplications. Said coatings have a large power of absorption of solarenergy and low emissivity characteristics in order to reduce energylosses through thermal radiation in the remote infrared region. Whatevertheir application, the selective absorbing coatings play an essentialpart.

There are two magnitudes denominated absorbance (α) in the UV_VIS(200-1000 nm) region and emittance (ε) in the infrared (1-15 μm) regionfor evaluating the efficiency of selective absorbing coatings. Thegreater a and the smaller e, the higher is the efficacy of the coating.

The selective coatings for the efficient absorption of solar energy andits conversion into heat are characterized because they have areflectance spectrum that changes abruptly in the wavelength where theintensity of solar radiation is nil (about 2 μm), from a very low value(about 5%) to a very high value (greater than 90%) above this wavelengthcorresponding to the infrared region of the spectrum. This ensures thatthe heat acquired by the metallic element is not lot lost throughthermal radiation.

Several patents and patent applications related to solar selectivecoatings are known. Usually, the coatings are made of a metal,dielectric or ceramic material substrate, at least one reflectingmetallic layer and at least one anti-reflection layer and their directapplication is in absorbing pipes for parabolic-trough solar collectorsand in absorbing sheets for solar panels, such as in patentsES2316321B2, ES2317796B2 and patent application WO2012172148A1. The mainadvantage is an absorbance greater than 95% and an emittance lower than0.2 in the temperature range from 400° C. to 550° C. However, theircompositions and methods of obtainment are very complex and thus wouldnot be economically sound in industries such as: food, textile, amongothers, because of their high production costs and thus the high priceof the final product.

Specifically, the inventions described in patents ES2317796B2 orES2316321B2, report very acceptable absorbance values but theiremittance values are not so favorable, leading to a selectivity ratioα/ε≦10.

Particularly, a large number of selective coatings have been describedthat use cermets formed by some of the following metals: Cu, Ni, Co, Pt,Cr, Mo, W, Al or Ag; and as ceramic matrix, the following compounds:SiO, SiO₂, Al₂O₃, AlN or MgO. In order to improve their efficacy, thesecermets must be covered with a layer of a material having very goodtransparent qualities such as the following oxides: Cr₂O₃, MoO₃, WO_(x),H_(f)O_(x) or SiO₂, where said layer acts as anti-reflection layer.Additionally, the cermet must be deposited on the metal acting asinfrared mirror, usually Ag, Cu, Al, Au or Pt.

Contrary to other patents, the working temperature of the instantinvention ranges from room temperature to 200° C. The manufacturingcosts are reduced and it is useful for use in other types of industriesand for applications where high working temperatures are not required.

The instant invention does not consist of multiple layers and very goodreflection and anti-reflection results are obtained simultaneously.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Cross section of a coating consisting of a substrate (1) ofmetallic material and metallic layer (2), in this example it is a crosssection of a coated metallic pipe. It represents the cross section of acoating consisting of a substrate (1) of metallic material and aradiation absorbing layer (2); and

FIG. 2. Reflectance value obtained with the coating.

DETAILED DESCRIPTION OF THE INVENTION

The instant invention is characterized because it consists of aradiation selective absorbing coating and a process for obtaining saidcoating.

The instant invention is characterized because it presents a procedurefor obtaining a solar radiation selective absorbing coating at roomtemperature using a stripping procedure.

Said coating consists of a substrate (1) of metallic material, that mayinclude, without limitation, dielectric or ceramic characteristics, andone single metallic layer (2) with reflection and anti-reflectioncharacteristics applied to the substrate (1) providing low emittanceproperties and having various uses, for example as solar selectiveabsorbent on metal surfaces or metal substrates, as well as thermosolarapplications, including, without limitation, in the food industry, inthe production processes, or a textile product such as a fabric orthread to be used in patches or inserts in the textile industry.

Said substrate (1) of metallic material may have surfaces of variousconfigurations and textures including, without limitation, smooth,rough, pipes, sheets, wires, filaments, spheres, etc.

For the above mentioned uses and applications, the solar radiationselective absorbing coating operates within a low and medium temperaturerange, from room temperature (25° C.) to 300° C., appropriate for beingused in devices generating heat through solar radiation or artificialillumination. Moreover, it is applied for heating water or any otherliquid.

The proposed invention also comprises a process contemplating at leastone cleaning stage, at least one first stage of immersion and standingin aqueous solution, at least one first rinsing stage, at least onesecond stage of immersion in aqueous solution and at least one secondrinsing stage.

The proposed invention relates to a procedure for obtaining a solarradiation selective coating using a stripping procedure, the procedurecontemplates at least one cleaning stage, one stripping process(immersion and standing in aqueous solution), at least one first rinsingstage, at least one stage of immersion in aqueous solution and at leastone second rinsing stage.

In said cleaning stage, the metal surface to be coated is cleaned withsolvents that include, without limitation, the following substances.

A mixture of silicates, phosphates, carbonates and sulfates, to removeimpurities such as dust and some greases;

Trichloroethylene for removing greases and oils that may be present onthe metal surface;

Acetone, for removing inorganic greases and polymer coverings that aredifferent from oxides.

After the cleaning stage, the substrate is immersed for the first timein a hydrofluoric acid aqueous solution at a concentration ranging from0% to 5% plus nitric acid at a concentration ranging from 5% to 15%.

After the cleaning stage, the substrate is submitted to a strippingprocess in a hydrofluoric acid aqueous solution at a concentrationranging from 0% to 5% plus nitric acid at a concentration ranging from5% to 15%. In a period of time from 8 to 16 minutes, the surface to becoated is allowed to stand immersed in the solution.

Then, the water rinsing stage is conducted (distilled water may beused).

Afterwards, in the second immersion stage, the substrate is immersedduring 9 to 13 hours in a chromic acid aqueous solution at aconcentration ranging from 200 g/L to 300 g/L and sulfuric acid at aconcentration ranging from 350 g/L to 450 g/L, obtaining an optimumcoating within 9.5 and 10.5 hours. This coating is generated in theindicated ranges at room temperature, between 20° C. and 40° C., and ata humidity ranging from 0% RH to 80% RH, because above this range waterprecipitation would be generated in the solution.

Then, the pre-treated substrate is immersed during a period of 9 to 13hours in a chromic acid aqueous solution at a concentration ranging from200 g/L to 300 g/L and sulfuric acid at a concentration ranging from 350g/L to 450 g/L, obtaining an optimum coating between 9.5 and 10.5 hours.This coating is generated applying the indicated ranges at roomtemperature, between 20° C. and 40° C., and at a humidity ranging from0% RH to 80% RH because above this range water precipitation would begenerated in the solution.

Finally, the substrate with the coating is withdrawn and is submitted toa rinsing stage that can be conducted with water or with an impurityremoving liquid.

Then, the metal substrate (1) is coated with one sole layer (2) ofchromium oxide having simultaneously reflecting and anti-reflectingcharacteristics.

The absorption level in the wavelength of 0.25 to 1.0 μm is 89%, thereflectance level in the wavelength of 2 to 15 μm is 21%.

The thickness of the layer of chromium oxide obtained is 200 nm.

The tests conducted on the selective absorbing coating with a typicalNi/NiO generate a high reflectance spectrum such as the one shown inFIG. 2.

As it can be observed, the instant invention has the advantage of beinga simple process that however has not been previously used for solvingsituations of cost reduction implemented in industries where processheat is required in the manufacturing process and where fossil fuels aremainly used, and it is thus considered a novelty for its simplicity andthe good technical results it permits to obtain.

Another advantage is that solvents and solutions necessary for obtainingit can be reused, optimizing thus the use of these inputs.

PREFERRED EMBODIMENT OF THE INVENTION

The procedure may use an additional polishing process in order toimprove the coating, considering that the sheets, pipes and spheres maybe polished; however, if this additional step is not used, such as inthe case of wires or metal fibers, this does not drastically reduce theabsorbance values.

The use of acetone is not mandatory is this procedure; this componentpermits to ensure the cleanliness of the metal substrate (1) but doesnot affect the obtained efficacy values.

Having clearly and sufficiently described my invention, I consider thatit is a novelty and claim as my exclusive property the contents of thefollowing claims:

1. A radiation selective absorbing coating comprising: a metalsubstrate; and at least one metallic layer; wherein said at least onemetallic layer possesses reflecting and anti-reflecting characteristics.2. A process for obtaining the radiation selective absorbing coatingaccording to claim 1, wherein the process includes the steps of: atleast one cleaning step; at least one stripping step; at least one firstrinsing step; at least one immersion step in an aqueous solution; and atleast one second rinsing step.
 3. The radiation selective absorbingcoating according to claim 1, wherein said metallic substrate havesurfaces of various configurations and textures, wherein the surfacesare selected from the group consisting of smooth, rough, pipes, sheets,wires, filaments, spheres, and mixtures thereof.
 4. The radiationselective absorbing coating according to claim 1, wherein said metalliclayer is made of chromium oxide.
 5. The process according to claim 2,wherein said steps are conducted at room temperature between 20° C. and40° C. and within a humidity ranging from 0% to 80% relative humidity.6. The process according to claim 2, wherein in the cleaning step, themetal surface is covered with solvents, the solvents are selected fromthe group consisting of a mixture of silicates, phosphates, carbonates,sulfates, trichloroethylene, acetone, and mixture thereof.
 7. Theprocess according to claim 6, wherein the cleaning step optionallyincludes acetone.
 8. The process according to claim 2, wherein in thefirst immersion step, the aqueous solution is hydrofluoric acid at aconcentration ranging from 0% to 5% plus nitric acid at a concentrationranging from 5% to 15%.
 9. The process according to claim 8, wherein inthe immersion step the surface to be coated is immersed during 8 to 16minutes.
 10. The according to claim 2, wherein the first and secondrinsing steps are conducted with water or with an impurity removingliquid.
 11. The process according to claim 2, wherein in a secondimmersion step, the aqueous solution is chromium acid at a concentrationranging from 200 g/L to 300 g/L and sulfuric acid at a concentrationranging from 350 g/L and 450 g/L and during 9 to 13 hours. 12.(canceled)
 13. The process according to claim 2, wherein the solventsand the solutions are reused.
 14. The radiation selective absorbingcoating according to claim 1, wherein the radiation selective absorbingcoating operates at a low and medium temperature ranging from 25° C. to300° C.
 15. The radiation selective absorbing coating according to claim1, wherein the coating is designed be applied to a device to generateheat through a solar radiation or an artificial illumination.
 16. Theradiation selective absorbing coating according to claim 1, wherein thecoating is used in thermosolar applications, the thermosolar applicationare selected from the group consisting of a food industry, productionprocesses, or a textile product including fabric or thread, and patchesor inserts in the textile industry.
 17. The radiation selectiveabsorbing coating according to claim 1, wherein the coating is use forheating water and liquids where a heat process heat is required.
 18. Theprocess according to claim 2, wherein pre-polishing process is notrequired.
 19. The radiation selective absorbing coating according toclaim 1, wherein level of absorption in a wavelength from 0.25 to 1.0 μmof 89%.
 20. The radiation selective absorbing coating according toclaim
 1. wherein the coating has a reflectance level in a wavelengthfrom 2 to 15 μm of 21%.
 21. The radiation selective absorbing coatingaccording to claim 1, wherein the thickness of the chromium oxide filmobtained is 200 nm.